fiber-optic cabling

Fiber-Optic cabling is a glass cabling media that sends network signals using light.

What is Fiber-Optic cabling?

A glass cabling media that sends network signals using light. Fiber-optic cabling has higher bandwidth capacity than copper cabling, and is used mainly for high-speed network Asynchronous Transfer Mode (ATM) or Fiber Distributed Data Interface (FDDI) backbones, long cable runs, and connections to high-performance workstations.

How Fiber-optic Works

Fiber-optic cabling consists of a signal-carrying glass core of 5 to 100 microns in diameter (a sheet of paper is about 25 microns thick and a human hair about 75 microns thick), surrounded by a layer of pure silica called cladding, which prevents light from escaping.

Surrounding the cladding are protective layers of acrylic plastic coating, Kevlar fibers for additional strength, and a PVC (polyvinyl chloride) jacket (usually colored a distinctive orange).

Network components use LED or laser diodes to convert electrical signals into light pulses for transmission on fiber-optic cables. An optical detector is used to convert the light pulses back into electrical signals.

Graphic F-5. Fiber-optic cabling.

There are two types of fiber-optic cabling:

  • Single-mode fiber-optic cabling:

    Has a narrow core (5 or 10 microns in diameter) and allows only one signal to be sent or received at a time over very long distances (up to 50 times farther than multimode fiber-optic cabling). Single-mode fiber-optic cabling uses laser-emitting diodes to introduce signals into the fiber and can transmit only one signal (light beam) at a time. Signal transmission is clear for approximately 30 miles (50 kilometers) before dispersion will distort signals, which means that single-mode fiber is ideal for long cable runs.

     

  • Multimode fiber-optic cabling:

    Has a thicker core (50, 62.5, or 100 microns in diameter) and has sufficient bandwidth to allow multiple signals to be simultaneously transmitted or received; each signal follows a different path or mode through the fiber. Light-emitting diodes are used instead of laser-emitting diodes to introduce signals into multimode fiber. Signal transmission is clear for approximately 3000 feet, but longer cable runs can distort signals through modal dispersion. There are two types of multimode fiber:

     

    • Step-index multimode fiber:
      The less costly variety of multimode fiber, it uses a wide core with a uniform index of refraction, causing the light beams to reflect in mirror fashion off the inside surface of the core by the process of total internal reflection. Because light can take many different paths down the cable and each path takes a different amount of time, signal distortion can result when step-index fiber is used for long cable runs. Use this type only for short cable runs.

       

    • Graded-index multimode fiber:
      The more expensive type of multimode fiber, it uses a core made of multiple concentric layers of glass, each having a lower index of refraction than the layer it contains. In graded-index fiber, light beams follow curved paths and all rays reach the end of the fiber simultaneously, reducing the signal distortion that occurs in step-index fiber when long cable runs are used.

       

Connectors for fiber-optic cabling come in several varieties, including SC, ST, and SMA connectors. ST connectors have a wider installed base, but SC connectors are more versatile and are becoming more popular. SMA connectors do not conform to EIA/TIA wiring standards.

Fiber-optic cabling has several advantages over copper cabling, including the following:

  • Data transmission rates of up to 100 Gbps and higher.
  • Low attenuation, allowing signals to be sent over distances measured in kilometers.
  • Less than 10 percent the weight of copper cabling.
  • Immunity to crosstalk and noise caused by electromagnetic interference (EMI).
  • Prevention of ground loops by electronically isolating transmitting and receiving stations.
  • Greater signal security because signals cannot be picked up by electromagnetic induction as they can from copper cabling. The fiber would have to be tapped (physically opened) in order to eavesdrop on the transmission.

Graphic F-6. Connecting two LANs using fiber-optic cabling.

Fiber-optic cabling is often used for campus-wide backbones, long cabling runs between buildings, and local area network (LAN) connections to heavily used servers or high-speed workstations. Fiber is used also in heavy industrial environments where machinery can cause high levels of EMI. Fiber is not used extensively at the LAN level yet because it is more expensive and more difficult to install than copper cabling. Long-distance telecommunications carriers such as Sprint and MCI use fiber-optic cabling exclusively for their country-wide telecommunications lines.

Different styles of fiber-optic cabling exist, depending on the intended use. Examples include the following:

  • Duplex multimode cable:
    With 62.5-micron core and 125-micron cladding, this is the most common general purpose cable type for most networking environments that require fiber-optic cabling. You can separate the two cables for termination purposes just as you can with an appliance power cord.

     

  • Breakout style cable:
    This style of cabling contains multiple individual simplex fibers that are stranded around a central strengthening member. It is used for work-area connections and does not require patch panels.

     

  • Distribution style cable:
    This style of cabling contains multiple individual simplex color-coded fibers that are stranded around a central strengthening member. It is often used in backbone applications that require multiple fiber connections.

     

  • Steel-reinforced cable:
    This type of cabling uses a corrugated steel inner tube for protecting cable that needs to be buried or run outdoors.

     

NOTE

Line drivers for fiber-optic cabling are available for synchronous or asynchronous transmission as well as for single-mode or multimode fiber, allowing you to extend or interconnect LANs in either point-to-point or multipoint configurations.

TIP

Remember that the bandwidth of a fiber-optic cable depends on the distance as well as the frequency. Bandwidth is usually expressed in frequency distance form, for example in MHz-km. In other words, a 500-MHz-km fiber-optic cable can transmit a signal a distance of 5 kilometers at a frequency of 100 MHz (5 x 100 = 500), or a distance of 50 kilometers at a frequency of 10 MHz (50 x 10 = 500). In other words, there is an inverse relationship between frequency and distance for transmission over fiber-optic cables.

To learn everything about this topic I recommend the book Cabling Part 2: Fiber-Optic Cabling and Components by Bill Woodward available on Amazon.

Be careful not to unduly stress fiber-optic cabling during installation. The maximum acceptable bend radius is usually 20 times the diameter of the cable. Use an optical time domain reflectometer (OTDR) to test for faults after installation. Loss of signal, or attenuation, in fiber-optic cables can be caused by absorption (no medium is completely transparent to light), cable microbending (especially in single-mode fiber if it is not installed correctly), connector loss because of poor splicing or poorly installed or misaligned connectors, or coupling loss at the transmitter or receiver.

For safety, never look down a fiber-optic cable connected to your network because the invisible laser light can injure the retina of your eye. When splicing connectors onto fiber, be careful to avoid getting shards of glass in your eyes or on your hands - use double-sided tape to clean the connection and remove loose shards. Wear protective eyewear.

Fiber-optic cabling is available for purchase in bulk for those who want the challenge of terminating it themselves, but most customers buy standard or custom preterminated cables from suppliers. These cables can be simplex or duplex; they can be single-mode or multimode (multimode is most common); and they can be terminated with ST-ST, ST-SC, SC-SC, or SMA connectors.